Batting him behind Johnson makes the end of the line up more vulnerable IMO. The idea of breaking up the hitters is to force the other teams pitchers to have to throw to new spots.
Johnson hits lefties just as well as righties. Anyway, I'm skeptical of this "new spots" theory. It seems to me, that the primary purpose of alternating left handed/right handed batters is to prevent the opposing manager from getting favorable lefty / righty matchups without burning through a new pitcher per batter.
